1. Field of the Invention
This invention relates to a printed circuit board that is suitable for high density packaging of various high-performance electronic apparatuses. The printed circuit board has excellent flexural rigidity or moisture absorption and repair resistance. This invention relates to also a method of manufacturing such a printed circuit board.
2. Description of the Related Art
Recently, various electronic elements composing electronic apparatuses have become smaller and thinner since electronic apparatuses have become small, thin, light-weight and better in the performance. Printed circuit boards for packaging these electronic elements also have been developed to provide a high density package.
With a rapid progress in packaging techniques, it is desired to supply circuit boards with a multilayer wiring structure at a low cost. Such a circuit board can be manufactured by mounting bare chips of a semiconductor device such as LSI directly at a high density, and it can correspond to a high-speed signal processing circuit. Such a multilayer circuit board is required to have high reliability in electric connection between wiring patterns of plural layers formed with a minute wiring pitch and also excellent high frequency property, and is required to have high reliability in a connection with a semiconductor bare chip.
For satisfying this requirement, a resin multilayer circuit board having an any-layer IVH structure is proposed in JP-A-6-268345. In this reference, a copper-plated conductor on a through-hole inner wall which mainly has been used for interlayer connection in a conventional multilayer circuit board, is replaced by filling of a conductor in a interstitial via hole (IVH) and at the same time, the IVH can be formed immediate below an element land or between arbitrary layers, so that substrate size can be decreased and a high density packaging can be provided. FIGS. 12A–12G show a method of manufacturing such a printed circuit board. First, as shown in FIG. 12A, mold-releasing films 401 made of polyester or the like are laminated on both surfaces of a porous substrate 402 such as an aramid epoxy pre-peg prepared by impregnating an aramid nonwoven fabric with a thermosetting epoxy resin. Next, as shown in FIG. 12B, via holes 403 are formed at predetermined positions of the porous substrate 402 by laser beam machining. Subsequently as shown in FIG. 12C, a conductive paste 404 is filled in the via holes 403. In this step, the porous substrate 402 with the via holes 403 is placed on a table of a screen printer, and the conductive paste 404 is directly printed on the mold-releasing film 401. The mold-releasing films 401 on the printing surface function as printing masks and prevent contamination of the surface of the porous substrate 402. Next, the mold-releasing films 401 are peeled off from the surfaces of the porous substrate 402, and then, metal foils 405 e.g., copper foils, are adhered to the surfaces of the porous substrate 402. By applying heat and pressure in this condition, the porous substrate 402 is compressed and thinned as shown in FIG. 12D. At that time, the conductive paste 404 in the via holes 403 also is compressed and binder ingredients contained in the conductive paste is extruded to strengthen bonding among the conductive ingredients and also between the conductive ingredients and metal foils 405. As a result, the conductive substance in the conductive paste 404 is condensed and the layers are connected electrically with each other. Subsequently, the conductive paste 404 and the thermosetting resin composing the porous substrate 402 are cured. The metal foils 405 are etched selectively to have a predetermined pattern so that a double-sided circuit board as shown in FIG. 12E is provided. Further, as shown in FIG. 12F, porous substrates 406 and metal foils 407 are stuck to the surfaces of the double-sided circuit board by using a conductive paste 408. After applying heat and pressure, as shown in FIG. 12G, the metal foils 407 are etched to have a predetermined pattern, so that a multilayer circuit board is obtained.
Resin multilayer substrates formed by using such substrates for forming circuits have been used for many electronic apparatuses because they have low coefficient of expansion, low dielectric constant, and light-weight.
However, a resin multilayer circuit board having the above-mentioned any-layer IVH structure has a core of an aramid nonwoven fabric, and a dielectric substrate is composed of a homogeneous mixture of an epoxy resin and aramid nonwoven fabric fibers. Such a dielectric substrate having a core of aramid nonwoven fabric has a coefficient of thermal expansion (CTE) of about 100 ppm/° C. in the thickness direction, which is different considerably from CTE (about 17 ppm/° C.) of an interstitial via conductor forming the any-layer IVH structure.
Since the properties of the electronic apparatus may deteriorate to some degree under a severe use condition with abrupt change in temperature, printed circuit boards with higher reliability have been desired.